Impact of Transient Environmental Conditions on Pitting and Cracking

Abstract

Coal-fired power stations that were originally designed to provide the baseload energy are now required to operate a two-shifting procedure; to better match consumer demand. The procedure involves switching between periods of on-load operation and off-load shutdown. This results in transient excursions in the chemistry of the steam due to the ingress of species such as oxygen and chloride. The synergistic combination of these species results in the initiation of corrosion pits which are known crack initiation sites in the corrosion fatigue (CF) process. Corrosion pits initiate in the low pressure steam turbine (LPST) of the energy extraction system, where the blades are commonly made from 12Cr stainless steel (SS). A major challenge in an industrial plant is the quantitative prediction of pitting, pit-crack transition and CF, defining inspection intervals and lifetime prediction of components and structures. By investigating the effects of transient environment conditions on corrosion pit initiation and observing the pit-crack transition, this PhD research project contributes towards understanding some of the challenges faced. Current research has utilised a full immersion setup to simulate a LPST by controlling the water chemistry and performing electrochemical measurements, such as corrosion potential monitoring. Further work is needed to understand the implications of environmental ageing on the development of the passive film, as there are results which suggest a superior passive film was developed as a result. By utilising additional electrochemical and characterisation techniques, such as electrochemical noise (EN) and X-ray photoelectron spectroscopy (XPS), this research supports claims that a more protective passive film forms, even after a short exposure to a simulated on-load environment. Characterisation of the passive films from different environments has shown that the composition is altered upon exposure to deaerated on-load conditions and provides an explanation for the increased chloride concentration required for pit initiation. Analysis of EN, with a bespoke MATLAB routine, was evaluated as an additional technique for monitoring the electrochemical processes occurring in the simulated LPST environments. An abundance of research has also shown that corrosion pits act as crack nucleation sites under an applied load, due to the concentration of stress and the impact of local electrochemistry. While most studies have focused on measuring the short and long crack growth rate from such defects, very few have looked at the early stages of crack initiation. More specifically, the location at which a crack nucleates at from a corrosion pit. Two main sites present themselves—the base and the mouth of the corrosion pit due to the concentration of stress and strain. X-ray computed tomography (XCT) work has previously shown that cracks preferentially initiated at the mouth for 3NiCrMoV steel under stress corrosion cracking (SCC) conditions, and this research has identified, with XCT and plasma focused ion beam (PFIB), that the same region is favoured for 12Cr martensitic SS under fatigue loading. The implications of this are that assumptions made about preferred crack nucleation at the base of a pit may not hold true in all cases.

Date of Award	6 Jan 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Fabio Scenini (Supervisor) & Brian Connolly (Supervisor)